MOLECULAR ASPECTS OF ENDOTHELIAL CELL FUNCTION AND HYPOXIA-DEPENDENT TUMOR ANGIOGENESIS

Abstract: Hypoxia, or deficient oxygen levels, is an inherent feature of rapidly expanding tumor tissue. In order to survive in the hostile, hypoxic environment and to restore the supply of oxygen and nutrients tumor cells activate a variety of adaptive mechanisms. Among them, hypoxia-induced angiogenesis, i.e. the formation of tumor blood vessels from pre-existing vasculature, has proved to be of critical importance, and has led to the development of anti-angiogenic therapy in the treatment of cancer patients. Anti-angiogenic drugs targeting the vascular endothelial growth factor (VEGF) pathway may prolong progression-free survival and overall survival in some patients. However, the results are more modest than predicted from pre-clinical studies, reflecting the complexity of tumor angiogenesis, and underlining the requirement of further studies aiming at the identification of the inherent limitations of this treatment as well as new targets of anti-angiogenic therapy. This thesis aims at investigating molecular aspects of hypoxia-dependent regulation of endothelial cell functions, and to identify new, potential targets of anti-angiogenic therapy. In the first part of the thesis, it is demonstrated that polyamines are potent modulators of endothelial cell biology. Accordingly, polyamines regulate both hypoxia-induced apoptosis in endothelial cells, through PI3K/AKT and MCL-1 dependent pathways, and sprouting angiogenesis via endothelial cell migration, partially explaining the anti-angiogenic effects of polyamine depletion in animal tumor models. During the initial steps of hypoxia-driven angiogenesis, endothelial cells respond to hypoxia by induction of a variety of adaptive mechanisms. Results presented in this thesis show that microRNAs seem to be involved in the endothelial cell adaptation to low oxygen levels, since hypoxia profoundly regulates the expression of a subset of miRNAs, and one of them, miRNA-424', has a functional role in hypoxia-dependent pro-angiogenic activity of endothelial cells. Finally, exosomes derived from aggressive brain tumor cells (glioblastoma) seem to be a novel, intercellular communication route eliciting a hypoxic, pro-angiogenic response in endothelial cells; a defined, hypoxic molecular profile of circulating, plasma-derived exosomes may serve as a non-invasive biomarker to assess the oxygenation status and aggressiveness of glioblastoma tumors. In conclusion, data presented in this thesis identify new players of hypoxia-mediated tumor angiogenesis and implicates them as potential targets of anti-angiogenic therapeutic intervention as well as biomarkers of cancer.